This invention relates to a method of spherically forming spherically a hemispherical shell having projections on its inner and outer surfaces and an apparatus for working said hemispherical shell, which are particularly suitably applied to a large-scale spherical pressure resisting shell for forming a control chamber in an abyssal diving research vessel.
In the abyssal diving research vessel for researching the bottom of an ocean or the like, since a crew embarks in a spherical pressure resisting shell acting as a control chamber, the shell has to have a hatch through which the crew has access to this pressure resisting shell, a viewing window or a hole through which various control cables penetrate or the like. Generally, while the pressure resisting shell is finished by interconnecting and welding a pair of hemispherical shells that are worked spherically, it is preferable for operability and prevention of the pressure resisting shell from an adverse thermal effect caused by welding that a flange for mounting the hatch on the hemispherical shell, a sash for the viewing window or the hole through which the cable penetrates or the like are worked before finishing said shell. In this case, to maintain the strength of the pressure resisting shell, the surroundings of said flange, sash portion or cable penetrated hole have to possess a wall thickness larger than the other portions. Thus, said hemispherical shell is provided with a large diameter hole for forming the hatch, viewing window or cable penetrated hole, and annular large thickness metal fittings providing the flange and sash or formed with the cable penetrated hole are fitted in and welded to said large diameter hole.
Now, when the inner peripheral wall of such a hemispherical shell is worked spherically and a portion of said thick metal fittings projects from the inner periperal wall of the hemispherical shell, a portion of connection between the hemispherical shell and the large thickness metal fittings provides a dead angle to a cutting tool in an existing three-dimensional cutting method so that such a portion cannot be cut since the projecting portion of the thick metal fittings interferes with the cutting tool. Thus, in conventional working methods before the thick metal fittings are fitted in the hemispherical shell and welded thereto, the inner peripheral wall of the hemispherical shell has been spherically worked and then the large diameter hole is provided in the hemispherical shell to fit and weld the thick metal fittings thereto. In such a working method, however, welding stress remains in the finished hemispherical shell so that the wall thickness of the pressure resisting shell has to be thicker than necessary to ensure a predetermined pressure resisting force. When the abyssal diving research vessel is adapted to travel in the abyss, it is important that the drive energy is minimized by counterposing the weight of said vessel to the buoyancy of same to improve the mobility of the vessel. Since in the present technology, however, the weight of the abyssal diving vessel is far larger than the buoyancy of same and further a great amount of buoyant materials needs to be mounted on said vessel, a large-scale and expensive vessel cannot be avoided.